U.S. Frost Dates by Weather Station

NOAA station USC00117354 · Illinois

Frost Dates for Rochelle, IL

NOAA weather station Rochelle (USC00117354), Illinois · elevation 775 ft (236 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 12only 1 spring in 10 still has a frost later
  • Typical last frostApril 2650% chance of a frost after it
  • Typical first fall frostOctober 1350% chance of a frost before it
  • Frost-free season168 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Rochelle, ILSpring: a frost after April 11 in 9 years out of 10, after April 26 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 13 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 168 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 26First frost ~Oct 13Frost season at Rochelle, ILSpring: a frost after April 11 in 9 years out of 10, after April 26 in half of the years, after May 12 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 13 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 168 days.JFMAMJJASONDLast frost ~Apr 26First frost ~Oct 13
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 27 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 12September 29
50% (typical)April 26October 13
90% (likely)April 11October 25
Frost-free season (32°F, 50% chance)
168 days
Hard frost (28°F), last in spring, 50% chance after
April 14
Hard frost (28°F), first in fall, 50% chance before
October 24

The last 15 years (2006–2020 normals)

Typical last frost April 29 (3 days later than the 30-year normal), typical first fall frost October 16 (3 days later). Frost-free season 170 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Rochelle: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 12, 20% May 6, 30% May 2, 40% April 30, 50% April 26, 60% April 22, 70% April 20, 80% April 16, 90% April 11.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 12Apr 26Apr 11Fall at Rochelle: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 4, 30% October 8, 40% October 11, 50% October 13, 60% October 16, 70% October 19, 80% October 22, 90% October 25.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 13Oct 25
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there.

What these dates mean

September 29 is the cautious fall date in Rochelle for finishing the harvest; only a 10 % chance of a light frost has occurred earlier, so tender crops can be pulled before the risk rises. The 50% light-frost dates—April 26 in spring and October 13 in fall—mark the point where a light frost is equally likely to appear before or after the date, giving gardeners a statistical midpoint for planning. A cautious plan for tender crops uses the May 12 spring date, which carries a 10 % chance of a later frost, and the September 29 fall date, which carries a 10 % chance of an earlier frost.

Hard frost, defined at 28 °F, can injure plants that would survive a 32 °F light frost, making the 50% hard-frost dates of April 14 and October 24 a more critical threshold for severe damage.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after May 12" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before September 29" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Rochelle, ILNumbered nearest stations: 1 — De Kalb, IL — 15 mi; 2 — Paw Paw, IL — 18 mi; 3 — Rockford, IL — 19 mi; 4 — Walnut, IL — 37 mi; 5 — Freeport, IL — 38 mi.WIIA25 mi · 40 km1. De Kalb, IL — 15 mi12. Paw Paw, IL — 18 mi23. Rockford, IL — 19 mi34. Walnut, IL — 37 mi45. Freeport, IL — 38 mi5
  1. De Kalb, IL 15 mi (25 km)
  2. Paw Paw, IL 18 mi (28 km)
  3. Rockford, IL 19 mi (31 km)
  4. Walnut, IL 37 mi (59 km)
  5. Freeport, IL 38 mi (61 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 41.9117, -89.0708.